Method and apparatus for the analysis of off-gases in a refining process



ATMOS July 14, 1970 R. FRUMERMAN 3,520,657

METHOD AND APPARATUS FOR THE ANALYSIS OF OFF-GASES IN A REFINING PROCESSANALYZER ANALYZE Filed Dec. 1965 INVENTOR. ROBERT FRUMERMAN UnitedStates Patent 3,520,657 METHOD AND APPARATUS FOR THE ANALYSIS OFOFF-GASES IN A REFINING PROCESS Robert Frumerman, Pittsburgh, Pa.,assignor to Dravo Corporation, Pittsburgh, Pa., a corporation ofPennsylvania Filed Dec. 27, 1965, Ser. No. 516,543 Int. Cl. C21c 7/00US. Cl. 23-230 14 Claims ABSTRACT OF THE DISCLOSURE Method and apparatusfor measuring the rate of flow of one constituent of a gas mixture andparticularly where the one constituent is evolved from a mass ofmaterial undergoing chemical and/ or physical change in a refiningprocess or the like. A tracer gas having a predetermined known rate offlow is injected into and admixed with the off gas and the resultingmixture analysed to determine the fraction of tracer gas and the oneconstituent and a proportion established in order to compute the rate offlow of the one constituent.

While the invention is applicable to a number of processes, it Will bedescribed specifically in connection with the refining of steel. Theterm refining in this specific context refers to the removal of carbonfrom the molten steel. Increasing demands are made on the steel industryto produce steel containing carbon within narrower limits than washeretofore acceptable. Under older practices, the carbon content of themelt was determined principally by the development of a practice over anumber of heats aided by time-consuming laboratory analysis of samplesfrom the heat. This was satisfactory with a wider range of permissiblecarbon content in the final product and in older refining processeswhich permitted time for sampling and laboratory analysis. The advent offaster refining processes, such as the oxygen blast process, and thenarrower permissible carbon limits requires new methods of carbonanalysis if full advantage is to be taken from the newer refiningmethods.

The present invention provides a means for quickly and accuratelydetermining the carbon content of the melt while refining isprogressing, whereby an operator may accurately determine the desiredend point. The specific embodiment illustrated is in connection with avacuum degassing operation Which in modern practices is generally thelast refining step before casting the melt. The described embodiment ofthe invention contemplates first measuring the carbon content and weightof the melt at the start of degassing using known analytical methods,then continuously measuring the weight of carbon removed in the efiluentgases and subtracting the latter quantity from the known initial weightof carbon to arrive at the carbon remaining in the melt. Substantiallyall the carbon removed from the melt is evolved as CO gas as is known inthe art. This basic technique has already been suggested but until nowthere has been no accurate and economically feasible method or apparatusfor analyzing the off-gases and relating the composition thereof to thecomposition of the melt nor is there any satisfactory way to directlyand rapidly measure the carbon content of the melt. Some of the majorproblems encountered in the off-gas analysis include the difficulty ofaccurately measuring flow rates and the fact that the pressure indegassing system varies so that no simple analytical instrument canaccept this variance without correction for pressure. Also, a gas sampleat high vacuum which is compressed to atmospheric pressure beforeanalysis, using a reasonable size of vacuum pump, be-

comes so small in volume that the time lag from pump discharge toanalysis make the measurement non-representative.

According to the invention, off-gases from the degassing vessel aredivided and a known small portion thereof passed through the samplesystem. Into the efifuent gases there is injected a tracer gas at aknown flow rate and the mixed off-gases and tracer gas are passedthrough two gas analyzers in a series at a controlled analyzer inletpressure. One analyzer continuously delivers an output signalrepresentative of the volume of mol fraction of CO in the sample and theother continuously gives an output signal representative of the volumeor mol fraction of tracer gas in the sample. A signal indicative of thetracer gas input flow rate is continuously divided by the tracer gasanalyzer output and the result continuously multiplied by the outputsignal from the CO analyzer to produce the instantaneous rate of COevolution. The latter quantity is recorded and integrated to determinethe total C evolved and this figure is subtracted from the initial Cdetermination to find the C content of the melt at any given time.Stated differently, the ratio of mol fraction to volumetric fiow ratefor each of the CO and tracer gases is equal so that the unknownquantity, CO flow rate, can readily be determined from this proportion.Suitable pumps and pressure regulators are provided in the system toprovide the necessary controlled flow conditions through the analyzers.

The invention utilizes the principle that the ratio of the partialpressures (mol fraction or other quantity derived from a volumetricanalysis) of two constituents in a gas mixture is proportional to theratio of the volumetric rates of flow of the two constituents into themixture. Thus if the partial pressures of each constituent can bemeasured and the volume rate of flow of one constituent is known, thevolume rate of flow of the other constituent can be calculated. Thisrelationship is true regardless of the varying composition of themixture. The partial pressure of a constituent is a function of thevolumetric rate of flow of that constituent into the mixture. Othersystem variables, such as total pressure and the presence of otherconstituents in the mixture, are inherently taken into account in theestablished proportion since these variables affect all the constituentsin the mixture in a proportionate manner. For the same reason, it is notnecessary to pass all the tracer gas through the analyzers once it hasbeen thoroughly mixed with the other constituents, and in fact in oneembodiment of the invention much of the tracer gas in the describedsystem for analyzing the efiluent during degassing is not passed throughthe analyzers, but is discharged through the ejectors.

The invention therefore provides a new and useful method and apparatusfor continuously determining the carbon content of a melt or fordetermining the quantity of a constituent remaining in a mass ofmaterial from which that constituent is evolved in gaseous form.

An object of the invention is to provide a new and useful method andapparatus for analyzing the gaseous etfiuent from a quantity of materialundergoing a physical and/ or chemical change.

Another object is to provide an improved method and apparatus forcontinuously determining the quantity of a constituent remaining in amass of material from which that constituent is evolved in a gaseousform.

Another object is to provide an improved method and apparatus forcontinuously determining the amount of carbon remaining in a melt duringa steel refining process.

Another object is to provide a method and a means for measuring the flowrate of one constituent of a mixture of gases flowing in a line, andparticularly where the one constituent varies as a proportion of themixture and the flow rate of the mixture also varies.

These and other objects will be apparent to those skilled in the art andmore fully understood by reference to the following description wherein:

The drawing schematically illustrates one embodiment of the novelapparatus for practicing the method of the invention.

Referring to the drawing, 10 is a container or ladle of moltencarbon-containing steel in the process of being vacuum degassed. Thedegassing vessel 11 illustrated is a refractory lined vessel of the typehaving two depending tubes 12 and 13 with their ends submerged in themetal bath. A lifting gas such as argon or the like is introduced to thetube 12 through line 14 to aid in flowing metal upwardly into the vessel11 Where degassing takes place. The degassed metal flows downwardlythrough tube 13 back to the ladle. Vacuum conditions in the vessel 11are created by ejectors 15 through line 16 communicating with the vessel11. The off-gases in line 16 from vessel 11 comprise a mixture of gasesincluding principally carbon monoxide and the lifting gas such as argon,smaller amounts of hydrogen and nitrogen, and negligible amounts ofcarbon dioxide and oxygen. A typical off-gas analysis (percent byvolume) at the start and finish of degassing, using argon as a liftinggas, might be as follows:

Start Finish Balance Balance 50 Line 16, before mentioned, has a smalllength to diameter ratio on the order of 1:5 in order to carry the largevolume of sub-atmospheric gases evolved from the metal in the degassingvessel. Paralleling line 16 is line 17 which has a large length todiameter ratio on the order of :1 and communicates with line 16 and theejectors through lines 18 and 19. Since streamline or laminar flowordinarily exists in lines 16 and 17 at the low pressures therein, theflow in line 17 will remain a known fixed proportion of the flow in line16. The mixture of gases carried by line 17 flows from junction 20 intwo directions, through line 19 as before described and to line 21, thenthrough vacuum pump 22, gas analyzers 23 and 24 and finally isdischarged to atmosphere by vacuum pump 25, the flow being in thedirection of the arrows in line 21. Various pressure control devices areplaced in line 21 to regulate the pressure to the analyzers at about 5mm. of Hg absolute.

The system shown is designed to supply a gas sample fioW through theanalyzers 23 and 24 of about one standard cubic foot per hour at aconstant pressure of about 5 mm. of Hg absolute. At this pressure theflow is equal to about two actual cubic feet per minute. The analyzersillustrated and preferred in this system are infrared gas analyzers. Thepressure level of 5 mm. is selected as a reasonable compromise since atthis pressure the volume flow of sample gas is about two cubic feet perminute which is a reasonable flow for the analyzers to handle andpressure control is relatively simple compared to pressures in thehigher vacuum range.

Since the pressure in the system, i.e., in line 17, varies between aboutone atmosphere down to 100 microns, two pressure control valves 26 and27 in parallel are used to maintain the pressure at the suction side ofpump 22 to a range of about 100 microns to 5 mm. Pressure control valve26 in line 21 is paralleled by pressure control valve 27 in line 28,which communicates with line 21 on both sides of valve 26. Both valves26 and 27 are controlled by pressure indicating controller 29, whichmonitors the pressure at the suction side of pump 22 and sends pneumaticsignals through lines 30 and 31 to the control valves 26 and 27,respectively.

Intermediate the discharge side of pump 22 and the analyzer 23, by-passline 32. is connected into line 21 and by-passes the analyzers 23 and24, being connected at its opposite end to line 21 at the suction sideof pump 25. Pressure control valve 33 in line 32 is controlled bypressure indicating controller 34 which senses the pressure at thedischarge side of pump 22 and sends a pneumatic signal via line 35 tovalve 33 to by-pass gases around the analyzers when the pressure at thedischarge of pump 22 exceeds 5 mm. of Hg absolute, the desired pressureinput to the analyzers. In line 21 downstream of line 32 and on theinput side of analyzer 23 there is a ditferential pressure cell 36 whichmeasures the flow into the analyzers and sends a pneumatic signal vialine 37 to flow recording controller 38 which in turn sends a pneumaticsignal via line 39 to flow control valve 40, which assists inmaintaining the pressure in the analyzers by controlling the flowthrough them.

Because dirt must ordinarily be filtered from the efiluent gases beforepassing through the analyzers a filter 22a is inserted in line 21upstream of pump 22 to prevent contamination of the analyzers. Asintered stainless steel cartridge type filter has been found suitable,but other types may be employed. The filter should be one which does notintroduce any appreciable additional time lag between sample point andanalyzers.

The sample gas temperature at the analyzers should preferably be aboutF. or lower. In the degassing installation illustrated this means thatthe gas temperature must be cooled from about 2900 F. down to 120 F. orlower. This heat is dissipated in the illustrated system by the pump 22.A Kinney model KDH-15O pump with a water jacket has been found suitablefor this purpose. If required a separate cooler may be installedupstream of the analyzers. Ambient temperature and the temperature tothe analyzers should not exceed 120 F. If higher ambient temperaturesare encountered it may be desirable to physically locate the analyzersin an air-conditioned enclosure.

The tracer gas system is indicated generally as 41 and comprises a gascylinder 42, which communicates with line 17 through line 43 andsuitable pressure and flow control devices. The tracer gas is preferablyintroduced at an elbow in line 17 to promote mixing with the off-gasesfrom the melt which are in laminar flow as before stated. The largelength to diameter of line 17 also aids the mixing process, and thesmall diameter of the line reduces the amount of tracer gas required toachieve the desired result. While other gases could be used, nitrousoxide is preferred because it is readily detectable by infraredanalyzers, is inexpensive and safe to use in this environment, and issubstantially non-reactive with the other gases known to be present. Thetracer gas pressure control system regulates the flow to about 4 to 5pounds per hour and comprises a valve 44, pressure control valves 45 and46 and flow control valve 47 all in line 43. Valve 46 is controlled bypressure recording controller 48 through pneumatic line 49. 50 is anelectric resistance heater to control the temperature of the tracer gasin line 43. In the described system a constant temperature of about F.has been found suitable. Intermediate valves 46 and 47 there is adifferential pressure cell 51 which continuously sends a pneumaticsignal indicative of the volume rate of flow of tracer gas to flowrecording controller 52 through line 53. The output of flow controller52 regulates valve 47 with a pneumatic signal via line 54 and this samesignal, which is proportional to the square of tracer gas velocitythrough differential pressure cell 51, is sent to the square rootextractor 55.

Referring back to the gas analyzers 23 and 24, analyzer 24 analyzes thesample gas flowing through it and continuously produces a pressureindicative electrical signal representative of the partial pressure ormol fraction of N 0 tracer gas in the sample. This electrical signal issent through lead 56 to a transducer 57 which converts the signal to apneumatic signal B which is sent via line 58 to divider 59 where it iscombined with the pneumatic signal A from the square root extractor 55via lead 60.

At the same time CO analyzer 23 continuously produces an electricalsignal representative of the partial pressure of CO in the sample streamand sends this signal via lead 61 to transducer 62 which converts it toa pneumatic signal C which is sent to a multiplier 63 via line 64. Inthe multiplier 63 the signal C is combined with the signal A/B fromdivider 59 -via line 65. The multiplier continuousl produces an outputsignal analogous to the computation A XC' where A is the rate of fiow ofN tracer gas in standard cubic feet per minute, B is the mol fraction ofN 0 tracer gas in the sample, and C is the mol fraction of CO gas in thesample, whereby the computation yields a signal D analogous to theinstantaneous rate of flow of CO gas in standard cubic feet per minute.The resultant signal D equal to A XC' is sent via line 66 to anintegrated flow recorder 67 and to an instantaneous flow recorder 68.Recorder 67 continuously integrates or totalizes the instantaneoussignals D to provide an indication of the total amount of CO evolvedfrom the start of the totalizing period. The totalized CO in the samplestream is readily convertible to totalized fiow from the melt since thesample stream fiow is a fixed known proportion of total flow from themelt. This figure is likewise readily convertible to total carbon (flowfrom the melt, and when subtracted from the initial carbon content ofthe melt, yields the remaining weight of carbon in the melt. The percentby weight of carbon in the melt can readily be calculated, and this isthe usual manner of expressing the carbon content.

The unique arrangement described regulates pressure in a manner toovercome the undesirable eifects of pressure variations in the mainstream and maintains the pressure to, and flow through, the analyzerssubstantially constant. Furthermore, even if the pressure at the inletto the analyzers vary slightly from the control point, the mathematicalutilization of the signals therefrom is such that the error due topressure variations is minimized. The reason for this is that theoutputs of the two analyzers are used as a ratio so that pressure errorstend to cancel each other out. If further pressure corrections should bedesired the analyzer outputs can be passed through an analogue computeror the like to correct for deviations in pressure from the controlpoint.

The conversion of the quantity of CO removed to the quantity of carbonremoved can be quickly calculated by the operator using a graph orspecial slide rule or the like. Likewise, the weight or percentage byweight of carbon remaining in the melt may be obtained by integratingthe instantaneous time rate of change of CO evolution using an analogcomputer or the like.

The gas analyzers preferably should be calibrated daily and for thispurpose there is provided an analyzer output indicating meter 69 whichselectively indicates the output of one or the other of the analyzers 23and 24. Switch 70 is selectively positionable to connect the pressuresignals from the analyzers to the indicating meter 69. Likewise thetracer gas bottle contents should be analyzed carefully and verified bythe use of a standard gas bottle which has been carefully analyzed Thedescribed embodiment of the invention thus comprises, by way of summary,a method and means for determining the carbon content of a melt of steelat any given time during the refining thereof, knowing the carboncontent and weight of melt at the start of the refining process. The useof a tracer gas injected into the sample stream of off-gases at a knownrate of flow of CO from the melt. The sample stream is analyzed for themol fraction of CO and tracer gas and the signals developed thereby arecombined with the flow rate of tracer gas to arrive at the flow rate ofCO.

In the described embodiment laminar flow is assumed to exist in lines 16and 17 and the tracer gas is introduced in line 17. In cases whereturbulent flow is encountered, the tracer gas may be introduced into thedegassing vessel itself and a sample stream taken ofi at some convenientpoint upstream of the ejector jets.

While the invention has been particularly described in connection withoff gas analysis in a metal refining process, it is apparent that itsusefulness may be extended to many other situations where it is desiredto measure the flow rate of a constituent of a gas mixture andparticularly where the flow rate of the mixture varies as Well as thequantity of the constituent in the mixture. In the degassing processdescribed, for example, pressure in the system and the gas flow may varyconsiderably during the process and the composition of the effluent gasmixture also varies considerably.

It will be apparent to those skilled in the art that variousmodifications of the method and apparatus described are possible withinthe scope and spirit of the invention.

I claim:

1. The method for determining the amount of carbon evolved from a massof molten metal subjected to a refining process wherein carbon iscontinuously evolved from the metal substantially entirely in the formof carbon monoxide gas, comprising (a) initially determining the carboncontent of the metal,

(b) confining the evolved carbon monoxide gas to floW in a line,

(c) continuously injecting a tracer gas into the line at a known flowrate to effect mixing thereof with the carbon monoxide,

(d) continuously generating a signal analogous to the tracer gas flowrate,

(e) continuously analyzing the mixture in the line and generatingsignals analogous to the fraction of carbon monoxide and the fraction oftracer gas in the analyzed mixture,

(f) continuously combining the generated signals to produce outputsignals indicative of the flow rate of carbon monoxide, and

(g) totalizing the output signals to find the total carbon evolved.

2. The method for determining the amount of carbon evolved from a massof molten metal in the form of carbon monoxide gas, wherein the carbonis evolved at a non-linear rate and in a mixture containing other gases,COIIIPIISlIlg,

(a) confining the evolved mixture of gases to How in a first line,

(b) dividing the flow to cause a known portion of the gases to fiow in asecond line having a substantially larger length to diameter ratio thanthe first line,

(c) continuously injecting a tracer gas into the second line at a knownflow rate and effecting mixing thereof with the evolved gas mixture toform a sample mixture,

(d) continuously analyzing the sample mixture to determine the fractionsof carbon monoxide and tracer gas in the sample mixture,

(e) continuously generating signals indicative of the tracer gas flowrate and the fractions of carbon monoxide and tracer gas in the samplemixture,

(f) continuously combining the generated signals in a manner to producean output signal indicative of the flow rate of carbon monoxide, and

(g) totalizing the output signals to find the total amount of carbonevolved.

3. The method as defined in claim 2, wherein the tracer gas is nitrousoxide.

4. The method as defined in claim 2, wherein the generated signals arecombined to produce the output signal by continuously solving theequation A B X C D where A is the rate of flow of tracer gas in volumeper unit of time,

B is the fraction of tracer gas in the sample mixture,

C is the fraction of carbon monoxide gas in the sample mixture, and

D is the rate of flow of carbon monoxide gas in volume per unit of time.

5. Apparatus for determining the rate of flow of a gas in a line,comprising (a) means for continuously injecting a tracer gas into theline at a known rate of flow,

(b) gas analyzing means in the line capable of continuously generatingsignals indicative of the instantaneous fractions of each of the gas andtracer gas passing therethrough,

(c) means for continuously generating signals indicative of theinstantaneous flow rate of tracer gas, and

((1) means for continuously combining the generated signals in a mannerto produce an output signal indicative of the flow rate of the gas.

6. Apparatus as defined in claim 5, wherein the gas analyzing meanscomprises two analyzers in series, one for each of the gas and tracergas.

7. Apparatus as defined in claim 6 wherein the two analyzers areinfrared analyzers.

8. Apparatus as defined in claim 5, including pressure regulating meansin the line, intermediate the point of injection of the tracer gas andthe gas analyzing means, for maintaining a predetermined pressure at theinput to the analyzing means.

9. Apparatus for determining the flow rate of a constituent of a gasmixture confined to flow in a line, comprising (a) a supply of tracergas,

(b) means for continuously introducing the tracer gas into the line toform a second mixture with the first mentioned mixture,

(c) a pair of gas analyzers in the line downstream of the point ofintroduction of tracer gas, which analyzers are arranged in series toreceive a flow therethrough of a regulatable sample stream portion ofthe total gas mixture flow, one analyzer being tuned to analyze themixture for the fraction of tracer gas therein and the other being tunedto analyze the mixture for the fraction of the constituent therein, bothanalyzers being adapted to continuously generate signals indicative ofthe instantaneous fractions of their respective analyzed gases,

((1) means of continuously generating signals indicative of theinstantaneous flow rate of tracer gas,

(e) means for regulating and controlling the amount of tracer gasintroduced,

(f) means in the line for regulating and controlling the flow throughthe analyzers, and

(g) means for combining the generated signals to continuously produceoutput signals indicative of the instantaneous flow rate of theconstituent.

10. Apparatus for determining the amount of carbon evolved from a bathof molten metal during a refining process wherein the carbon iscontinuously evolved in a gaseous state from a container of metal,comprising (a) a first line in which the evolved gas is confined toflow,

(b) a second line smaller than and communicating with the first line andadapted to conduct a sample stream portion of the gas, which portion isa known proportion of the flow in the first line,

(c) a container of tracer gas,

(d) means for continuously introducing the tracer gas into the samplestream at a known flow rate,

(e) means for continuously generating signals indicative of the tracergas instantaneous flow rate,

(f) a pair of gas analyzers arranged in series in the second line andthrough which the sample stream and tracer gas flow, one of theanalyzers being adapted to continuously generate signals analogous tothe instantaneous fraction of evolved carbon flowing therethrough andthe other analyzer being adapted to continuously generate signalsanalogous to the instantaneous fraction of tracer gas flowingtherethrough,

(g) means for continuously combining the generated signals tocontinuously generate output signals analogous to the instantaneous rateof carbon evolution, and

(h) means for continuously totalizing the output signals to compute thetotal amount of carbon evolved from the start of the totalizing period.

11. In a vacuum degassing installation having a container ofcarbon-containing molten metal to be degassed, a vacuum vessel throughwhich the metal is flowed and while fiowing therethrough carbon isevolved in the form of carbon monoxide gas along with other gases in amixture, and an outlet duct in which the evolved gases are confined toflow, apparatus for measuring the amount of carbon evolved from the meltin the form of carbon monoxide gas, comprising,

(a) a pipe line communicating with the outlet duct and so arranged thata known portion of the evolved gas mixture flows therethrough,

(b) means for introducing a tracer gas into the evolved mixture toeffect mixing thereof with the evolved mixture to form a sample streammixture,

(c) means for regulating the introduction of tracer gas to a known flowrate, and

((1) means communicating with the pipe line for continuously analyzingthe sample stream to continuously determine the respective instantaneousfractions of carbon monoxide and tracer gas flowing in the samplestream,

(e) means for continuously generating signals indicative of tracer gasfiow rate and the fractions of tracer gas and carbon monoxide,

(f) means for continuously combining the generated signals to generatean output signal indicative of the instantaneous flow rate of carbonmonoxide, and

(g) means for totalizing the output signals to indicate the amount ofcarbon monoxide evolved.

12. Apparatus as defined in claim 11 including means in the pipe linefor filtering dirt from the evolved mixture before the mixture flowsthrough the analyzing means.

13. Apparatus as defined in claim 11 including means for controlling thetemperature of the gas mixture flowing into the analyzing means.

14. In combination with a container of carbon containing molten steel, avacuum degassing vessel communicating with the metal, means foreffecting flow of the metal into the vessel, an outlet ductcommunicating with the vessel for carrying away gases evolved from themetal, which gases contain carbon monoxide in a mixture with other gasesfrom the metal, apparatus for measuring the amount of carbon monoxideevolved from the melt over a period of time, comprising (a) a first pipeline communicating with the outlet duct and so arranged that a knownportion of the evolved gases fiow therethrough,

(b) a supply of tracer gas different from any of the other gases in themixture which are present in substantial quantity and beingsubstantially nonreactive with the other gases in the mixture,

(c) a second pipe line communicating with the first pipe line and thesupply of tracer gas for introducing tracer gas into the evolvedmixture,

(d) a pressure control valve in the second line for regulating thepressure of the tracer gas,

(e) a flow meter in the second line for continuously measuring the rateof flow of the tracer gas,

(f) a flow control valve in the second line,

(g) a flow-regulating controller connected to the flowmeter and the flowcontrol valve for regulating the flow control valve in accordance withthe flow meter indication and for continuously generating signalsindicative of the instantaneous rate of flow of tracer (h) a third pipeline communicating with the first and second lines and through whichflows the tracer gas and evolved gas mixture in a sample stream.

(i) a pressure control valve, a first vacuum pump, a pair of gasanalyzers, and a second vacuum pump, in the third line in series in theorder named, and in the direction of flow of the sample stream, arrangedto maintain a substantially constant sample stream flow through andpressure in the analyzers, one of the analyzers being adapted tocontinuously generate signals indicative of the instantaneous fractionof tracer gas in the sample stream, the other References Cited UNITEDSTATES PATENTS 3,096,157 7/1963 Brown et a1. 3,181,343 5/1965 Fillon7323 XR 3,329,495 7/1967 Ohta et al 73-23 XR OTHER REFERENCES Walker etal., Principles of Chemical Engineering (1927), pp. 23-24.

MORRIS O. WOLK, Primary Examiner R. E. SERWIN, Assistant Examiner U.S.Cl. X.R.

